What is the difference between `mut` before a variable name and after the colon in Rust?

Hi everyone! I recently started learning Rust and encountered some confusion with variable declaration syntax. In tutorials, I see two different ways of using the `mut` keyword, and I can't understand the difference between them. The first way — `mut` is placed before the variable name: ```rust let mut x = 5; ``` The second way — `mut` is placed after the colon when specifying the type: ```rust let x: mut i32 = 5; ``` I tried both options in my code, but the second one causes a compilation error. Here's an example I wrote: ```rust fn main() { let mut a = 10; // works a += 5; let b: mut i32 = 20; // error! b += 10; println!("a = {}, b = {}", a, b); } ``` The compiler says something about "expected type, found keyword mut". I thought